This invention relates generally to high voltage transmission lines and more particular to a gas insulated transmission line having a particle trap including deflector means.
A gas insulated transmission line typically comprises a grounded metal sheath, a high voltage conductor within the sheath, insulators maintaining the conductor spaced from the sheath, and a compressed gas such as sulfur hexafluoride between the conductor and the sheath for the purpose of maintaining a high dielectric strength therebetween. It has been recognized, though, that loose solid conductive or semiconductive particles in the compressed gas can impair its dielectric strength and also the dielectric strength along the surface of the insulator. To eliminate the problem caused by these particles, various means have been utilized to remove or deactivate them. U.S. Pat. Nos. 3,515,939 -- Trump; 3,792,188 -- Cronin; 3,813,475 -- Cronin; and 3,898,367 -- Nakata are various means utilized in the prior art to deactivate these particles. All these patents describe a method of providing a low or zero field region near the outer metallic sheath as a means of entrapping the particles. The particles which move in the electrical field and may travel back and forth between the outer sheath and the inner conductor eventually find their way into the low field region, where they are trapped. The low field region traps these particles because the force acting on the particle which would cause it to leave the outer sheath and travel through the gas to the inner conductor is equal to the product of the particle's induced charge and the electric field intensity. However, in the low field region, the electric field intensity is low, which reduces the force causing the particle to leave by a factor equal to the square of the reduction in the electric field intensity.
However, a problem may arise during operation of the transmission line. A high voltage DC current may be impressed upon the typically alternating current being transported along the inner conductor. This DC field may cause the particle to raise off of the outer sheath and travel between the outer sheath and the inner conductor, thereby possibly initiating electrical breakdown. Also, the particles may become dislodged from the low field region by a mechanical shock acting on the outer sheath. This mechanical shock could cause the particles to re-enter the higher field regions, where they may be detrimental to the transmission line. A third manner in which such particles may become dislodged is if, upon installation of the system after initial conditioning of it, the transmission line section is installed incorrectly. If, for example, the transmission line is installed with the trap at the vertical top of the line, the force of gravity may cause the particle to leave the low field region and possibly initiate breakdown. Therefore, it may be desirable to provide some means of insuring that such particles entering low field regions, or particle traps, remain in the traps irregardless of whether mechanical shock, gravity, or direct current fields act upon such particle.